2000 Outback

First off, yes you can, that is if you're willing to get used/refurbished parts and do it yourself. Thats what i would do, but then again thats how i grew up. Secondly, the gear ratios are set for a balance of economy and performance, and I'm assuming you're in 5th/6th gear at these revs on the highway (at least i hope you are). You can get adjusted gear ratios for whatever you use this car for from the STi tune shop or any other performance tuning company. Back to 1, getting used/ refurbished parts isn't bad in the least, and since the Outback was a popular car, you should have no trouble finding the part you need, I would do it myself (WITH A MECHANIC BY YOUR SIDE IF YOU HAVEN'T DONE THIS BEFORE SO YOU DON'T MUCK IT UP!) because it allows you to learn how to work on your car, which is handy if you break down in the middle of nowhere without cell phone service or civilization for miles. So, in conclusion, buy it used, do it yourself, and econ/performance is balanced by Subaru engineers.

well you can do it for cheaper but you CAN NOT buy a used catalytic converter, it's against the law for a used one to even be sold from a parts yard or such. I replaced the cat conv on my Subie not to long ago with a Magnaflow direct fic cat conv and it was very easy took 1/2 hour tops, however if you do this you will want to replace the gaskets that get exposed in the exhaust system where it gets seperated to do your fix or your exhaust will not seal properly when put back together. Now as for the high rpms that's about what a Subaru should run to keep adequate power to sustain highway speeds, it may be a little high but that could be the sighn of a bad clutch that is sliping, how many miles are on the car and when was the last time the clutch was replaced?

if it is the code P0420 it may be other things besides the Cat, like the O2 sensor maybe a hole in your exhaust system I would do a little more investigaing before spending that kind of money